Name: periodic trends in atomic size
Uploaded: 2020-09-03T13:38:28.000Z
Duration: 3 min 8 s
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K12 Standards

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North Carolina K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.2.2

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Atomic Radii and Effective Nuclear Charge

Bir atomun boyutu elektronlar veya orbitalleri tarafından belirlenir. Bununla birlikte, orbitaller sınırlı bir alanı tanımlamaz, daha çok bir elektronun nerede bulunabileceğinin istatistiksel olasılığını tanımlar. Öyleyse atom büyüklüğü nasıl tanımlanır ve nelerden etkilenir?Bir atom yarıçapı iki şekilde tanımlanabilir. Bağ yapmayan atom yarıçapı veya bir atomun van der Waals yarıçapı, atomik katıdaki bitişik çekirdekler arasındaki mesafenin yarısıdır. Tersine, bağ yapan atomik yarıçapı veya kovalent yarıçap, metaller ve ametaller için farklıdır.Metallerde, kristal yapılarındaki atomlar için yarıçap, iki komşu atomun merkezleri arasındaki mesafenin yarısı olarak tanımlanır. Ametal, iki atomlu moleküllerde yarıçap, bağlı atomların merkezleri arasındaki mesafenin yarısı olarak tanımlanır. Periyodik tablo, genellikle atom yarıçapları olarak adlandırılan ve iki faktörden etkilenen kovalent yarıçaplardaki varyasyonları gösterir;değerlik elektronlarının temel enerji seviyelerinin sayısı ve efektif nükleer yük.Sütunların altındaki ana grup elementlerinin atom yarıçapındaki trend burada görülür. Bir grup aşağıya inerken, temel kuantum sayısı n, her element için bir artar. Böylece, dış elektronlar çekirdekten uzaklaştıkça atom yarıçapı grup içinde artar.Örneğin, grup 1'in altlarına inerken atom yarıçapı lityumdan sezyuma doğru artar. Bu trend, tüm periyodik tabloda görülmektedir. Ayrıca, grafik atom yarıçapının her alkali metal için maksimum olduğunu ve periyod boyunca her soy gazla minimuma düştüğünü ortaya koymaktadır.Bir periyod boyunca azalan atom yarıçapları, efektif nükleer yük ile açıklanabilir. Efektif nükleer yük kavramını hatırlayalım. Herhangi bir çok elektronlu atomda, iç kabuk elektronları, çekirdeğin çekiminden dış kabuk elektronlarını kısmen perdeler.Bu nedenle, efektif nükleer yük, yani dış elektronun hissettiği yük, gerçek nükleer yükten daha azdır. Aynı değerlik kabuğundaki elektronlar birbirlerini çok etkili bir şekilde perdelemez. Periyod boyunca, iç kabuk elektronlarının sayısı sabit kalırken nükleer yük artar.Böylece, etkili nükleer yük giderek artarken, dış elektronların koruması azalır ve bu, atom yarıçapında bir azalmaya yol açar. Bununla birlikte, çoğu geçiş elementinin yarıçapı her sırada neredeyse sabittir. Bunun nedeni, en dıştaki temel enerji seviyesindeki elektron sayısının neredeyse sabit olmasıdır.

atomik yarıçaplar ve etkili nükleer yük

Periyodik tablonun gruplarındaki elementler benzer kimyasal davranış sergilerler. Bu benzerlik, bir grubun üyelerinin değerlik kabuklarında aynı sayıda ve aynı dağılımına sahip elektronlarının olması nedeniyle oluşur.
 
 Periyotta soldan sağa gidildikçe, çekirdeğe bir proton ve her ardışık elementle valans kabuğuna bir elektron eklenir. Elementlerin elektronik yapısının anlaşılması, kimyasal davranışlarını yönlendiren bazı özellikleri incelememize izin verir. Bu özellikler, elementlerin elektronik yapısı değiştikçe periyodik olarak değişir. 
 
 <h4>Atomik Yarıçapta Varyasyon</h4>
 
 Kuantum mekaniksel çerçevede, bir atomun belirli bir boyutunu belirlemeyi zorlaştırır. Bununla birlikte, atomların yarıçapını tanımlamanın ve böylece kabaca benzer değerler veren göreceli boyutlarını belirlemenin birkaç pratik yolu vardır.
 
 Metallerdeki atom yarıçapı, iki komşu atomun merkezleri arasındaki mesafenin yarısıdır. Bu, diatomik moleküller olarak var olan elementler için bağlı atomların merkezleri arasındaki mesafenin yarısıdır.
 
 Periyotta soldan sağa hareket edildikçe, genellikle, her bir elementin kendisinden önceki elementten daha küçük bir atom yarıçapı olduğu görülür. Bu mantıksız görünebilir çünkü daha fazla elektrona sahip atomların daha küçük bir atom yarıçapına sahip olduğunu ima eder. Bu, etkili bir nükleer yük kavramına dayanarak açıklanabilir. Herhangi çok elektronlu atomda, iç kabuk elektronları kısmen dış kabuk elektronlarını çekirdeğe çekilmesinden korur.  Bu nedenle, etkili nükleer yük, bir elektron tarafından hissedilen yük, gerçek nükleer yükten (Z) daha azdır ve aşağıdaki gibi hesaplanır:
 
 Zeff = Z – σ              
 
 burada, Zeff efektif nükleer yük, Z gerçek nükleer yük ve σ ekranlama sabitidir, burada ekranlama sabiti sıfırdan büyük ancak Z'den küçüktür.
 
 Bir periyot boyunca bir elementten diğerine her geçtiğimizde, Z bir artar, ancak ekranlama sadece biraz artar. Böylece, Zeff bir periyot boyunca soldan sağa doğru hareket ettikçe artar. Periyodik tablonun sağ tarafındaki elektronların yaşadığı daha güçlü çekme (daha yüksek etkili nükleer yük), onları çekirdeğe yaklaştırır ve atom yarıçaplarını daha küçük hale getirir.
 
 Çekirdek elektronlar, en dıştaki ana seviyedeki elektronları nükleer yükten etkili bir şekilde korur, ancak en dıştaki elektronlar birbirlerini nükleer yükten etkili bir şekilde korumazlar. Etkili nükleer yük ne kadar büyük olursa, çekirdeğin dış elektronlar üzerindeki tutumu o kadar güçlü olur ve atom yarıçapı o kadar küçük olur.
 
 Bununla birlikte, bazı geçiş elementlerinin yarıçapları her satır boyunca kabaca sabit kalır. Bunun nedeni, en dıştaki ana enerji seviyesindeki elektronların sayısının neredeyse sabit olması ve kabaca sabit etkili nükleer yük yaşamasıdır.
 
 Her periyotta, atom yarıçapındaki eğilim Z arttıkça azalır;  her grupta, eğilim, atom yarıçapının Z arttıkça artmasıdır.
 
 Bir grubu tararken, ana kuantum sayısı, n, her eleman için bir artar. Böylece, elektronlar çekirdekten giderek daha uzak olan bir alan bölgesine eklenmektedir. Sonuç olarak, en dıştaki elektronların çekirdekten olan mesafesini arttırdıkça atomun boyutu (ve atom yarıçapı) artmalıdır. Bu eğilim, aşağıdaki tabloda halojenlerin atom yarıçapları için gösterilmiştir.
 
 <table style="margin-left: auto; margin-right: auto;">
 <tbody style="border: 1px solid black;">
 <tr>
 <td colspan="3" style="color: black; border: 1px solid black; text-align: center;">Halojen Grubu Elementlerinin Atomik Yarıçapları</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">Atom</td>
 <td style="color: black; border: 1px solid black; text-align: center;">Atom yarıçapı (pm)</td>
 <td style="color: black; border: 1px solid black; text-align: center;">Nükleer yük, Z</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">F</td>
 <td style="color: black; border: 1px solid black; text-align: center;">64</td>
 <td style="color: black; border: 1px solid black; text-align: center;">9+</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">Cl</td>
 <td style="color: black; border: 1px solid black; text-align: center;">99</td>
 <td style="color: black; border: 1px solid black; text-align: center;">17+</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">Br</td>
 <td style="color: black; border: 1px solid black; text-align: center;">114</td>
 <td style="color: black; border: 1px solid black; text-align: center;">35+</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">I</td>
 <td style="color: black; border: 1px solid black; text-align: center;">133</td>
 <td style="color: black; border: 1px solid black; text-align: center;">53+</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">At</td>
 <td style="color: black; border: 1px solid black; text-align: center;">148</td>
 <td style="color: black; border: 1px solid black; text-align: center;">85+</td>
 </tr>
 </tbody>
 </table>
 
  
 
 Bu metin bu kaynaktan uyarlanmıştır: <a href="https://openstax.org/books/chemistry-2e/pages/6-5-periodic-variations-in-element-properties">Openstax Chemistry 2e, Section 6.5: Periodic Variations in Element Properties.</a>

Atomik Yarıçaplar ve Etkili Nükleer Yük

Periyodik tablonun gruplarındaki elementler benzer kimyasal davranış sergiler. Bu benzerlik, bir grubun üyelerinin aynı numaraya sahip olması nedeniyle oluşur ...

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Periodic Properties of the Elements

Elementlerin Periyodik Özellikleri

KEY TERMS AND CONCEPTS

TEMEL TERİMLER VE KAVRAMLAR

periodic trends in atomic size

Periodic trends in atomic size help explain why atoms get larger or smaller across the periodic table. Elements in the same group show similar chemical behavior because they have the same number and arrangement of valence electrons. Across a period, each new element adds one proton to the nucleus and one electron to the valence shell. Down a group, the number of valence electrons stays the same, but the principal quantum number increases by one.

Atomic radius is not easy to define with perfect precision because atoms do not have a hard edge. Scientists use practical measurements instead. In metals, atomic radius is one-half the distance between the centers of two neighboring atoms. For elements that form diatomic molecules, it is one-half the distance between the centers of bonded atoms.

Moving from left to right across a period, atomic radius generally decreases. That may seem surprising because atoms on the right have more electrons. The reason is effective nuclear charge, or the charge an electron feels from the nucleus. It can be estimated as Z eff = Z – σ, where Z is the actual nuclear charge and σ is the shielding constant.

As Z increases by one from element to element across a period, shielding rises only a little. This means Z eff also increases from left to right. A stronger pull from the nucleus draws outer electrons closer, so the atomic radius becomes smaller. Core electrons shield outer electrons well, but outermost electrons do not shield one another very effectively. Some transition elements keep nearly the same radius across a row because the number of electrons in the outermost principal energy level changes very little, so the effective nuclear charge stays roughly constant.

The trend within a group is the opposite. Atomic radius increases as Z increases down a group. Each step down adds a principal energy level, so the outermost electrons are farther from the nucleus. That greater distance makes the atom larger. The halogens show this pattern clearly, with atomic radii increasing from fluorine to astatine.

Periodic Trends in Atomic Size

periodic classification of the elements

How Valence Electrons Shape the Periodic Table

The periodic table organizes elements by increasing atomic number, and it also shows why elements in the same group behave alike. As the electron configurations are placed into the table, a repeating pattern appears in the outer shells of atoms. These outer electrons are called valence electrons, and they play the biggest role in chemical reactions. They are the highest-energy electrons in an atom, so they are easier to lose or share than core electrons. Valence electrons also help determine...

ionization energy

Ionization Energy Trends in the Periodic Table

Ionization energy is the energy needed to remove an electron from a gaseous atom or ion in its ground state. The first ionization energy, or IE1, removes the most loosely bound electron and forms a 1+ cation. The second ionization energy, IE2, removes the next electron, and each later ionization energy removes another electron from the atom or ion. Ionization always requires energy, so these processes are endothermic. That means ionization energy values are always positive. In general, larger...

atomic radii and effective nuclear charge

Periodic trends in atomic size help explain why atoms get larger or smaller across the periodic table. Elements in the same group show similar chemical behavior because they have the same number and arrangement of valence electrons. Across a period, each new element adds one proton to the nucleus and one electron to the valence shell. Down a group, the number of valence electrons stays the same, but the principal quantum number increases by one. Atomic radius is not easy to define with perfect...

ionic radii

How Ionic Charge Changes Ion Size

Ionic radius describes the size of an ion. It changes when an atom gains or loses electrons. Cations are positive ions with fewer electrons than the parent atom, and they are smaller than the atoms they come from. Aluminum shows this pattern clearly. A neutral aluminum atom has a covalent radius of 118 pm, while Al3+ has an ionic radius of 68 pm. When electrons are removed from the outer valence shell, the remaining core electrons feel a stronger effective nuclear charge, or Zeff. That pull...

electron affinity

Electron Affinity Trends on the Periodic Table

Electron affinity is the energy change that happens when a gaseous atom gains an electron and becomes a negative ion, or anion. For many elements, this process releases energy, so the electron affinity value is negative. For some elements, energy must be added instead, so the value is positive. Electron affinity also applies when more than one electron is added. The second electron affinity is the energy change for adding an electron to an anion to make a 2− ion. Later electron affinities...

alkali metals

Alkali Metals: Reactivity Trends

Alkali metals are Group 1 elements with one valence electron. They are soft, shiny metallic solids that are malleable, ductile, and good conductors of heat and electricity. Their melting points are unusually low for metals and drop as you move down the group. Density generally increases down the group, except for potassium. The trend in atomic size helps explain their behavior. Moving from lithium to cesium, each element has a higher principal quantum number, so the outer electron is farther...

halogens

Halogens: Trends and Reactions

Halogens are Group 17 nonmetals that show clear trends in structure and reactivity. At room temperature, fluorine and chlorine are gases, bromine is a liquid, and iodine is a solid. Astatine is highly unstable and radioactive, so most of its properties are still unknown because it has a very short half-life. Tennessine is a synthetic element that is also predicted to belong to this group. These elements do not usually exist as single atoms. Instead, they form diatomic molecules, which means...

noble gases

Noble Gases: Sources, Uses, and Reactions

Noble gases are the group 18 elements helium, neon, argon, krypton, xenon, and radon. They were called “noble” because they were once thought to be nonreactive. That idea changed in 1962, when Dr. Neil Bartlett at the University of British Columbia showed that this assumption was not fully correct. These gases occur in very small amounts in the atmosphere. Some natural gas contains 1–2% helium by mass. Helium is separated from natural gas by removing the condensable components, which leaves...

key terms and concepts

Learn why atomic radius changes across periods and groups, and how effective nuclear charge and shielding shape the size of atoms in the periodic table.

Periodic Trends in Atomic Size | JoVE K12

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Periodic Table Trends: Atomic Radii, Ionization, and Electronegativity

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Radioactive Decay and Radiometric Dating

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The Energies of Atomic Orbitals

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The Aufbau Principle and Hund's Rule

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The alkali metal sodium (atomic number 11) has one more electron than the neon atom. This electron must go into the lowest-energy subshell available, the ...

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Molar Mass

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PS.Chm.2.1

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Ionic Radii

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Ionization Energy

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Electron Affinity

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Construct an explanation to infer the atomic size, reactivity, electronegativity, and ionization energy of an element based on its position in the Periodic Table.

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Analyze and interpret data to explain the mechanisms and properties of the two main types of intramolecular (ionic and covalent) bonds.

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Analyze the structure of atoms and isotopes.

Periodic Trends in Atomic Size

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